RoGeorge, thanks for asking:
Please also see, DODGY SECTION, first page, has Optical Bench Redux.
Those are cheapo Solar Garden Lights, each having a small, shorter version of an AAA rechargeable. They have a 4-pin control IC, that, obviously, has some kind of voltage comparator, front end, that switches an inverted output,...at 222 khz near square wave.
So, never mind, that it's AC output, I just call that an 'ON' state, and seems like input is also a light lift, as that 222 khz isn't seen (as AC), but rather, a simple, raw solar cell just gives a steady DC on (1.2 to 2.0 volts in direct sun), so I'm considering that as similar enough, to DC in DC out ...(even while that's not, technically, true.)
Lots and lots of possibilities, like a 'Liquid Suspension' module, no wires, no power (no external pwr wires), and using a quite powerful, optically linked, component.
Smart 'dust', self-organizing, likely proposed many years ago.